146 



PURPLE BACTERIA. 



92. Influence of the Individual Colours of the 

 Spectrum. 



The necessity for a separate consideration of these red species, 

 grouped together by ENGELMANN (V.) as purple bacteria, is soon 

 apparent when an attempt is made to study their physiology. The 

 bacteria described in the previous chapter, of which Micrococcus 

 prodigiosus may serve as an example, also produce red colouring 

 matter ; but, in their case, the latter is a mere inert waste product, 

 appearing under certain conditions, or absent under others, without 

 the growth of the cell being thereby seriously affected. Contrary 



to this, the purple bacteria do not 

 excrete any colouring matter into 

 the environment, but the pigment 

 occurs exclusively within the cells 

 (Fig. 39) in the part of the contents 

 which immediately adjoins the cell- 

 wall, and which is described in 

 Chapter ii. as the parietal layer. 

 The bacterio-purpurin is not always 

 distributed throughout the whole of 

 this layer, but is frequently restricted 

 to isolated spots therein, and, in 

 exceptional instances, is altogether 

 absent. It is not present in any 

 definite shape, such as granules, 

 bands, or plates, like the chlorophyll 

 of the higher plants, but occurs 

 diffused in the plasma. 



It has already been stated in 

 Chapter iii. that the purple bacteria 

 exhibit great avidity for light, and 

 therefore always strive towards the 

 sunlight. Closer observation shows 

 this behaviour to be intimately connected with the presence of 

 bacterio-purpurin. It was a happy thought on the part of Engel- 

 mann to investigate the nature and extent of the influence exerted 

 on the vitality of the purple bacteria by the several colours of the 

 spectrum. 



If a preparation rich in such organisms be placed in a drop of 

 water, and a micro-spectrum of a few millimetres in length be pro- 

 jected thereon, a rapid movement towards certain parts of the 

 spectrum will be observed under the microscope, the organisms 

 collecting and resting there in macroscopically visible bands. By 

 quickly killing the cells they will remain in position and constitute 

 a permanent preparation, or, as Engelmann appropriately named 

 it, bacterio-spectrogram. This, when submitted to examination, 

 is found to correspond with the absorption spectrum of bacterio- 



FlG. 39. Chromatium Okenii. 

 Optical longitudinal section. 



The cell-wall (the thickness of which 

 is somewhat exaggerated in the 

 figure) immediately adjoins the 

 outer layer of protoplasm, which 

 carries the colouring matter 

 shown as fine dots. Large sul- 

 phur granules are present in the 

 interior of the cell. Magn. about 

 1600. (After F. Forster.) 



